The present invention relates to a metallized polyimide film in which a metal layer of a metal such as copper is formed on the surface of a polyimide film, and a production method thereof, and relates particularly to a metallized polyimide film used as a TAB tape, a flexible circuit board, or a flexible wiring board or the like.
In recent years, the demand for circuit boards using TAB (Tape Automated Bonding) or FPC (Flexible Print Circuit), which offer advantages in the trend towards smaller, lighter weight and structurally more flexible electronic devices, has continued to increase. Conventionally, these types of circuit boards have used a flexible plastic substrate with a layer of copper foil bonded to this substrate using an adhesive such as an epoxy based adhesive.
However, in order to increase the packaging density in electronic devices, it is desirable to reduce the film thickness of this type of circuit board even further, and the structure described above, in which copper foil is bonded to a substrate, is unable to fully satisfy these demands for even thinner films.
Furthermore, in the aforementioned circuit boards using an adhesive, (1) the etching solution for the copper foil may penetrate through to the adhesive layer, and when a bias is then applied under conditions of high temperature and high humidity, copper migration may occur, causing a short circuit within the circuit, (2) in order to improve speed, the impedance must be matched and cross-talk reduced, but the presence of the adhesive makes this difficult, (3) the dimensional stability of the adhesive layer is poor, (4) the presence of the adhesive layer makes ultra fine processing of the circuit board difficult, making a shift to higher densities problematic, (5) the thermal characteristics of the adhesive layer are inferior to those of the plastic substrate material, leading to problems of thermal stability, and making a shift to higher densities problematic, and (6) the presence of the adhesive means that deformation of the product is more likely to occur.
In order to resolve these problems, techniques for forming a metallized film without using an adhesive are being investigated. Known examples include methods in which thin film formation techniques such as vacuum deposition, sputtering or ion plating are used to form a metallic thin film directly onto a plastic substrate in accordance with a circuit pattern, and a metal plating layer is then deposited on top of this metallic thin film using electroplating or the like, and methods in which a metallic thin film is formed on the surface of a plastic substrate, a metal is then deposited on top of this thin film using electroplating or the like, and the metal layer is then etched to form a circuit pattern.
However, in these types of structures, the post processes such as the circuit pattern formation process and the electroplating process cause a reduction in the bonding strength between the plastic substrate and the metallic thin film, increasing the possibility of the metallic thin film separating from the substrate.
In order to resolve this problem, a configuration is disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 1-133729, in which a film of a zirconium oxide or a silicon oxide is formed on the surface of a polyimide film, and a copper layer is then formed thereon.
In Japanese Unexamined Patent Application, First Publication No. Hei 3-274261, a configuration is disclosed in which a film of nickel, chromium, titanium, vanadium, tungsten or molybdenum or the like is formed on the surface of a polyimide film, and a copper layer is then formed thereon.
In Japanese Unexamined Patent Application, First Publication No. Hei 5-183012, a configuration is disclosed in which a thin film of nickel, cobalt, tungsten or molybdenum or the like is formed on the surface of a polyimide film using electroless plating, and a copper layer is then formed thereon using electroplating methods.
In Japanese Unexamined Patent Application, First Publication No. Hei 7-197239, a configuration is disclosed in which a metal such as nickel, chromium, molybdenum or tungsten is deposited on the surface of a polyimide film using vacuum deposition, and a copper layer is then formed thereon using electroplating.
In Japanese Unexamined Patent Application, First Publication No. Hei 8-330695, a configuration is disclosed in which a thin film of molybdenum is formed on the surface of a polyimide film using sputtering, and a copper layer is then formed thereon using electroplating.
However, none of the above methods is able to completely prevent the separation of the copper layer from the polyimide film, and a configuration capable of further strengthening the bonding between the copper layer and the polyimide film has been keenly sought.
The present invention takes the above circumstances into consideration, with an object of providing a metallized polyimide film which is capable of increasing the bonding strength between the metal layer and the polyimide film, as well as a method of producing such a metallized polyimide film.
In order to achieve the aforementioned object, a metallized polyimide film according to the present invention comprises a polyimide film which has undergone surface roughening treatment to produce a surface Ra value of 2 to 10 nm, an intermediate layer formed from one, or two or more films selected from a group consisting of molybdenum, silicon and silicon monoxide, which is formed on top of the surface which has undergone surface roughening treatment with an average thickness of 5 to 50% of the aforementioned Ra value, and a conductive metal layer which is formed on top of the intermediate layer. The thickness of the conductive layer will vary depending on the material used in the metal layer and the film formation method employed, although it is though that typically the thickness will be at least 300 angstroms, and at least 1000 angstroms as the film becomes more dense.
According to this type of metallized polyimide film, a suitable degree of surface roughening treatment is performed on the surface of the polyimide film, and a film of molybdenum, silicon or silicon monoxide is then formed thereon with a suitable distribution, and consequently the bonding strength of the metal layer can be improved.
A method of producing a metallized polyimide film according to the present invention comprises a step for performing surface roughening treatment on the surface of a polyimide film to produce a surface Ra value of 2 to 10 nm, a step for forming an intermediate layer from one, or two or more films selected from a group consisting of molybdenum, silicon and silicon monoxide, which is formed on top of the polyimide film surface which has undergone surface roughening treatment with an average thickness of 5 to 50% of the aforementioned Ra value, and a step for forming a metal layer of a conductive thickness on top of the intermediate layer.
The surface roughening treatment described above can utilize at least one of alkali etching treatment, vacuum plasma treatment, and atmospheric corona treatment.
A metallized polyimide film of the present invention may also be in the form of a TAB tape, a flexible circuit board, or a flexible wiring board or the like.